HIV-1, the predominant cause of AIDS in humans, is unable to replicate in most non-human species. Therefore, the most practical animal model of AIDS consists of infection of rhesus macaques with SIVMAC or chimeras derived from SIVMAC that express the HIV-1 envelopes (SHIV). However, both of these models have specific limitations. In the case of SHIV, most chimeras developed to date use the CXCR4 co-receptor (X4-tropic) in contrast to most primary HIV-1 isolates that use the CCR5 co-receptor (R5-tropic). The search for R5-tropic SHIVs that replicate and cause disease in animals consistently has been much sought after goal. In this proposal we aim to use a novel approach towards the generation of such SHIVs. Based on our experience in manipulating viral sequences we have optimized a semi-automated procedure that will allow the generation and screening by infectivity of a large number of R5-SHIVs. Furthermore, we will use Env-coding sequences from viruses that have recently been identified as being transmitted and establishing infection in human hosts, the precise target of potential interventions. We will test the ability of our SHIVs to replicate in vitro. Replication competent viruses will be mixed and cocktails of viruses will be used to inoculate animals and allow in vivo selection of viruses with the highest replicative capacity. Ultimately, we will generate SHIVs that closely resemble HIV-1 strains that circulate in humans and replicate efficiently in an animal model. If successful, these studies should revolutionize the preclinical exploration and development of AIDS therapeutics and vaccines.